Aids to the teaching of music, whether vocal or instrumental, and for the tuning of instruments have, in the past generally employed one or the other of two concepts. In accordance with one of these concepts the tone identifying device is comprised of a plurality of tone generators each of which may be individually activated to generate a tone of known frequency. A tone originating in a musical instrument or in the vocal cords of a voice trainee is fed into the device through a microphone and amplifier and is applied to comparison circuitry, such as a cathode ray tube, along with the tone that is being generated as selected within the device. Identification of the tone that is being generated externally and applied to the device is then accomplished through a comparison and matching technique which is at best an indirect method providing no useful output until a match has been accomplished and therefore not giving a direct indication of tone identity nor lending itself readily to the identification of two or more tones simultaneously sounded nor the possibility of a steady and lasting indication of the externally generated tone when the production of that tone has ceased.
The other concept involves the employment in the tone identifying device of a plurality of tuned vibratory elements such as reeds or tuning forks. These are tuned to the frequencies of the notes of the musical scale and are arranged to be set in sympathetic vibration either accoustically or electrically, in response to an externally generated tone that is to be identified and that is introduced into the device through a microphone or other type of transducer. Petroff U.S. Pat. No. 2,779,920, granted Jan. 29, 1957, is an example of an audio-frequency meter employing tuned reeds. A laminated iron core is arranged to influence all of the tuned reeds magnetically and the core carries a winding upon which are impressed the tone signals that are to be identified. In one embodiment the reeds are arranged to serve as light shutters and in another embodiment they are arranged to complete, in their vibratory excursions, the circuits of lamps.
The tone display device employing tuning forks is disclosed in Balamuth U.S. Pat. No. 3,204,513, granted Sept. 7, 1965. In this disclosure tones to be individually identified by the lighting of lamps are applied to a plurality of windings each associated with one of a set of tuning forks to set the tuning fork in vibration at its natural frequency through an iron core associated with the winding. Each tuning fork also has associated with it a pickup coil to be energized at the frequency at which the tuning fork is vibrating. Each of the pickup coils controls a circuit closure device for completing a circuit of a lamp individual to the pickup coil and therefore representing the tuning fork that has been set in vibration.
Tone identifiers employing mechanically vibrating tone identifying components have significant shortcomings. Considering first the problems involved in the employment of reeds, they are likely to be position-sensitive, by which is meant that they may be more responsive to a tone stimulus in one position, such as a vertical position, than they are in another position, such as a horizontal position. Moreover, minor changes in position, such as due to jarring, or major changes, such as due to tilting of the structure supporting the reeds, is likely to produce erroneous indications. Another disadvantage in the use of vibrating reeds is that they are subject to a hysteresis-like manifestation. The reeds have been found to respond more effectively when activated in an ascending sound spectrum sequence than they do when activated in a descending sequence. Reeds cannot be made to exhibit a high Q, which is a figure of merit related to a factor of rejection of frequencies below and above the frequency that they are intended to pass. Moreover, they cannot be produced practically to exhibit desired values of Q at any frequency.
Reeds have poor dynamic range because sensitivity is limited at the low frequencies and linearity can be achieved only over a very narrow range. The inertia that characterizes mechanical vibratory devices introduces another undesirable effect when those devices are employed as tone responsive components of a tone identifying apparatus, the undesirable effect being that the tone must be sustained until vibration of the reed or fork builds up to useful amplitude. At higher frequencies reeds become very short and the amplitude of displacement becomes so small that operation of light shutters or opening and closing of electrical contacts cannot be accomplished. Moreover, vibratory objects such as reeds are subject to fatigue in the material of which they are made, and the result is that a tone identifier employing reeds that is subjected to substantially constant usage, as in the training of music students or in speech therapy, may require undesirably frequent replacement of reeds. As a corollary of this, electrical contacts used to open and close electrical circuits do not have unlimited useful or effective life, among the limiting factors being wear and other degradation, such as pitting. When the contacts are reeds vibrating at voice frequencies the limit may be reached in a very short time, a matter of a few hours. It is not unusual for electrical contacts to fail after making and breaking a few million times, perhaps ten million. A vibratory reed tuned to operate at 440 cycles per second, which is the frequency of A above middle C in the musical scale, reaches ten million operations in a few hours.